Type 2 diabetes (T2DM) develops in 50% of women with gestational diabetes (GDM) within 5 years after delivery. So far, no lifestyle interventions have been translated for use in women with GDM during the early postpartum period. The screening for GDM practiced in the health system presents a highly efficient identification of a population at high risk of T2DM. The primary aim of the study is to implement and evaluate a diet and physical activity (PA) intervention tailored to postpartum mothers and feasible for a health plan setting, with the primary goals of: a) reaching pre-pregnancy weight if women had a normal pre-pregnancy weight; or b) achieving a 5% reduction from pre-pregnancy weight if overweight or obese before pregnancy. The primary outcomes are the proportion of women who reach their postpartum body weight goal and the total amount of weight change. Secondary outcomes are plasma glucose levels, percent calories from fat, total calories and time spent in PA. Participants will be from a large population (66% from ethinic minority groups) receiving medical care at Kaiser. Women will be randomly assigned to lifestyle intervention or usual medical care (175 in each arm). The interventionists will be dieticians trained in social cognitive strategies and motivational interviewing techniques and located at the Kaiser Perinatal Service Center. The intervention will be delivered via individual telephone contacts and brief in-person counseling sessions augmented with e-mails and a website. Goals for the intervention during pregnancy (Phase I) are to help women comply with the IOM guidelines for weight gain. Phase I will be delivered through 1 individual in-person session and 2 telephone counseling contacts. Phase II begins at 6-weeks postpartum and will be delivered through 2 individual in- person sessions and 10 telephone counseling contacts over a period of 4 months, after which Phase III (maintenance) will begin and participants will receive 9 tailored diet and PA reminders until 24-months postpartum. Intervention women will also be taught about their diabetes risk and how to prevent it. Measurements will be collected at study entry and at 6-wks, 6-m, 12-m, 18-m and 24-m postpartum. Data analyses will be by intent-to-treat. Tertiary aims address psychosocial modeling, within trial cost-effectiveness analysis and evaluation of the intervention for possible adoption. A finding that this intervention is cost-effective in this setting should motivate changes in the health system that would facilitate its adoption.